+1 on the Coolfire 4. It's a great mod for anyone. What type of tank are you using on it?
Nicotine
A "general guideline" for nicotine concentration in e-liquid to the number of cigarettes smoked per day. If you are subohming (resistance under 1.0Ω) consider cutting these nicotine levels by at least 50%.
06 mg = 10 cigarettes or less
12 mg = 20 cigarettes
18 mg = 30 cigarettes
24 mg = 40 cigarettes
eLiquid/Juice
PG = Propylene Glycol - the "flavor carrier" of e-liquid. A secondary source of "throat hit", but not as strong as the nicotine concentration. Some people have a sensitivity or even an allergy to PG. It's very thin.
VG = Vegetable Glycerin - the "vapor maker" of e-liquid. Used as a sweeter in many food sources, and also as a skin emollient. It's very thick. Often labeled just "Glycerine"
PG and VG are often used together in different ratios in e-liquid. For example, a 50/50 PG/VG ratio. Some juice vendors offer a range of different ratios as far as PG/VG.